The NTSB concluded they crew overstressed the aircraft when recovering from a loss of control. However the aircraft had neither CVR or FDR, nor was it required to (the CVR had been removed when converted for cargo use), hampering the investigation.
NTSB Report Summary
As the wings failed, the propellers simultaneously chopped through the fuselage behind the cockpit [and] the horizontal stabilizers...also failed upward.
Wreckage:
